Battery electrode, method for making the same and hybrid energy storage device using the same

ABSTRACT

The present invention relates to a battery electrode. The battery electrode comprises a plurality of carbon nanotubes and a plurality of transition metal oxide nanoparticles. The plurality of transition metal oxide nanoparticles are chemically bonded to the plurality of carbon nanotubes through carbon-oxygen-metal (C—O-M) linkages, wherein the metal being a transition metal element. The present invention also relates a method for making the battery electrode and a hybrid energy storage device using the battery electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201810298532.6, filed on Apr. 3, 2018, inthe China National Intellectual Property Administration, the contents ofwhich are hereby incorporated by reference. This application is relatedto applications entitled, “CARBON NANOTUBE-TRANSITION METAL OXIDECOMPOSITE AND METHOD FOR MAKING THE SAME”, filed Apr. 2, 2019 Ser. No.16/373,093.

FIELD

The present disclosure relates to a battery electrode, a method formaking the battery electrode and a hybrid energy storage device usingthe battery electrode.

BACKGROUND

Lithium-ion batteries and supercapacitors are common energy storagedevices. Lithium-ion batteries typically provide a high energy density.However, a cycle life of lithium-ion batteries is short and a powerdensity of lithium-ion batteries is low. On the contrary, thesupercapacitors character in long cycle life and high power density, butthe supercapacitors are limited for their low energy density. Therefore,it is highly desirable to develop a single electrochemical energystorage device to achieve the advantages of battery-like energy andsupercapacitor-like power simultaneously.

Transition metal oxides have been intensively studied as promisingcandidates of electrode materials in lithium-ion batteries to satisfymarket demand of high power density due to their capacitive-likelithium-ions storage behaviors and high theoretical capacitance.However, an application of transition metal oxides for high energydensity has been hampered by their poor electronic conductivity, limitedlithium-ion diffusion coefficient, slow reaction kinetics and poorstructural stability during cycles.

In order to solve the above problems of the transition metal oxides, ithas been proposed to use nano-scale electrode particles to formhigh-power lithium-ion batteries. The use of nano-scale electrodeparticles comprises the following advantages: first, a diameter ofnanoparticles tends to be relatively small, so a diffusion path oflithium-ion can be significantly shortened; second, the ultrathinnanoparticles can mechanically buffer a volume change during acharge/discharge process, thus an electrode integrity can be maintainedand a cycle stability can be strengthened. At present, a binder such aspolyvinylidene fluoride (PVDF) is often used to bond transition metaloxides and conductive agents together to prepare an ultrathin material,and the ultrathin material is used as an electrode. However, thetransition metal oxides usually have a large size (greater than 50 nm).The transition metal oxides is poorly dispersed in the electrode and iseasy to agglomerate. The van der Waals force between the transitionmetal oxides and the binders is weak. Therefore, the transition metaloxides can easily fall off from the electrode, resulting in a sharp dropof battery capacitance.

What is needed, therefore, is to provide a battery electrode, to solvethe problems discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the exemplary embodiments can be better understood withreference to the following drawings. The components in the drawings arenot necessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the exemplary embodiments.Moreover, in the drawings, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 shows a flow chart of a method for making a battery electrodeaccording to one embodiment.

FIG. 2 shows transmission electron microscope (TEM) images of a pristineCNT, an air-CNT and an MnO₂/aCNT, respectively.

FIG. 3 shows an X-ray diffraction (XRD) pattern of an MnO₂/aCNTcomposite film structure.

FIG. 4 shows an Mn 2p X-ray photoelectron spectroscopy (XPS) diagram ofthe MnO₂/aCNT composite film structure.

FIG. 5 shows diagrams of nitrogen adsorption-desorption isotherm of theair-CNT and the MnO₂/aCNT composite film structure; and FIG. 5 insetshows pore size distribution diagrams of the air-CNT and the MnO₂/aCNTcomposite film structure.

FIG. 6 shows a TEM image of the MnO₂/aCNT composite film structure.

FIG. 7 shows a photograph of the MnO₂/aCNT composite film structure.

FIG. 8 shows a Raman spectra of the air-CNT, an MnO₂ powder and theMnO₂/aCNT composite film structure, respectively.

FIG. 9 shows carbon (C) is and K 2p XPS diagrams of the air-CNT and theMnO₂/aCNT composite film structure.

FIG. 10 shows oxygen (O) is XPS diagrams of the MnO₂ powder, the air-CNTand the MnO₂/aCNT composite film structure, respectively.

FIG. 11 shows a thermogravimetric analysis (TGA) curve of the MnO₂/aCNTcomposite film structure.

FIG. 12 shows scanning electron microscope (SEM) images of a rCNT, theair-CNT, a MnO₂ electrode, a MnO₂/rCNT electrode and a MnO₂/aCNTelectrode, respectively.

FIG. 13 shows discharge and charge curves of the MnO₂ electrode, theMnO₂/rCNT electrode, and the MnO₂/aCNT electrode, respectively.

FIG. 14 shows cyclic performance of the MnO₂ electrode, the MnO₂/rCNTelectrode, the air-CNT electrode and the MnO₂/aCNT electrode at acharge/discharge current of 0.2 Ag⁻¹.

FIG. 15 shows rate tests of the MnO₂ electrode, the MnO₂/rCNT electrodeand the MnO₂/aCNT electrodes at a constant discharge rate of 0.5 Ag⁻¹.

FIG. 16 shows long cycle tests of the MnO₂/aCNT electrode at a highcharge/discharge current of 2 Ag⁻¹ and 5 Ag⁻¹, respectively; and FIG. 16inset shows discharge/charge voltage profiles curves of the 2^(nd)cycle, the 500^(th) cycle, and the 1000^(th) cycle of the MnO₂/aCNTelectrode, respectively.

FIG. 17 shows kinetic analysis of the MnO₂/aCNT electrode; FIG. 17(a)shows cyclic voltammetry (CV) curves at different scan rates from 0.2 to20 mV s⁻¹; FIG. 17(b) shows b-value determination at differentpotentials around current peaks; FIG. 17(c) shows CV curve at 20 mV s⁻¹.FIG. 17(d) shows Mn 3s XPS diagram before and after discharge.

DETAILED DESCRIPTION

The disclosure is illustrated by way of embodiments and not by way oflimitation in the FIGures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent FIGures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale, and the proportions of certain parts havebeen exaggerated to illustrate details and features of the presentdisclosure better.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape, or other feature which is described, suchthat the component need not be exactly or strictly conforming to such afeature. The term “include,” when utilized, means “include, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

A battery electrode is provided according to one embodiment. The batteryelectrode comprises a plurality of carbon nanotubes and a plurality oftransition metal oxide nanoparticles. The plurality of transition metaloxide nanoparticles are chemically bonded to the plurality of carbonnanotubes through carbon-oxygen-metal (C—O-M) linkages, and the metalbeing a transition metal element.

A diameter of each of the plurality of carbon nanotubes is not limited.The plurality of carbon nanotubes can be a plurality of single-walledcarbon nanotubes or a plurality of multi-walled carbon nanotubes. In oneembodiment, the plurality of carbon nanotubes are a plurality ofmulti-walled carbon nanotubes. The plurality of carbon nanotubes areentangled and closely combined with each other to form a free-standingfilm structure. The term “free-standing structure” implies, but is notlimited to, that the carbon nanotube film structure can sustain theweight of itself when it is hoisted by a portion thereof without anysignificant damage to its structural integrity. The free-standing filmstructure can serve as a conductive network and an attachment carrierfor the transition metal oxide nanoparticles.

The plurality of transition metal oxide nanoparticles can be distributedin the battery electrode uniformly. The transition metal oxide is notlimited and can be selected from a group consisting of manganese dioxide(MnO₂), titanium dioxide (TiO₂), ferroferric oxide (Fe₃O₄), chromicoxide (Cr₂O₃), cobaltosic oxide (Co₃O₄), molybdenum dioxide (MoO₂),vanadium dioxide (VO₂), and combinations thereof.

Surfaces of the plurality of carbon nanotubes can define a plurality ofpores. The plurality of transition metal oxide nanoparticles aredeposited on surfaces of the plurality of carbon nanotubes or in theplurality of pores.

Because the plurality of carbon nanotubes are entangled with each otherto form a freestanding structure. The freestanding structure is highconductive and flexible. In addition, the plurality of transition metaloxide nanoparticles are bonded to the plurality of carbon nanotubesthrough chemical bonds and are not easily detached from the carbonnanotubes. Therefore, the battery electrode need no current collector,conductive agent, and binder, so that a weight and a size of the batteryelectrode can be reduced. In one embodiment, the battery electrode onlyconsists of the plurality of carbon nanotubes and the plurality oftransition metal oxide nanoparticles. In another embodiment, the batteryelectrode further comprises a small amount of structural water and metalions. The metal ions can be alkali metal ions such as potassium ions(K+). The metal ions and the structural water can stabilize the batteryelectrode during cycles and maintain a cycle performance of the batteryelectrode.

In the present disclosure, the carbon nanotubes and the transition metaloxide nanoparticles are tightly boned together through C—O-M chemicalbonds. The chemical bonds can prevent the transition metal oxidenanoparticles from falling off the carbon nanotubes at a high charge anddischarge current.

Referring to FIG. 1, a method for making the battery electrode comprisesthe following steps:

step 1, providing a plurality of carbon nanotubes obtained from asuper-aligned carbon nanotube array;

step 2, pre-oxidizing the plurality of carbon nanotubes;

step 3, dispersing the plurality of carbon nanotubes in a solvent toform a first suspension;

step 4, dispersing a material containing transition metal oxyacidradicals in the first suspension to form a second suspension; and

step 5, removing the solvent from the second suspension and drying thesecond suspension to obtain the battery electrode.

The step 1˜5 are described in detail as followings.

In the step 1, the plurality of carbon nanotubes is provided. Theplurality of carbon nanotubes is obtained from a super-aligned carbonnanotube array.

A diameter and a length of each of the plurality of carbon nanotubes arenot limited. The plurality of carbon nanotubes can be a plurality ofsingle-walled carbon nanotubes or a plurality of multi-walled carbonnanotubes. In one embodiment, the plurality of carbon nanotubes are aplurality of multi-walled carbon nanotubes in order to prevent thepre-oxidized carbon nanotubes from being broken in the step 4.

The super-aligned carbon nanotube array is prepared by a chemical vapordeposition method, and the method comprises the following steps: (a)providing a substantially flat and smooth substrate; (b) forming acatalyst layer on the substrate; (c) annealing the substrate with thecatalyst layer in air at a temperature ranging from about 700° C. toabout 900° C. for about 30 to 90 minutes; (d) heating the substrate withthe catalyst layer to a temperature ranging from about 500° C. to about740° C. in a furnace with a protective gas therein; (e) supplying acarbon source gas to the furnace for about 5 to 30 minutes and growingthe super-aligned carbon nanotube array on the substrate.

In step (a), the substrate can be a P-type silicon wafer, an N-typesilicon wafer, or a silicon wafer with a film of silicon dioxidethereon. In one embodiment, a 4-inch P-type silicon wafer is used as thesubstrate.

In step (b), the catalyst can be made of iron (Fe), cobalt (Co), nickel(Ni), or any alloy thereof.

In step (e), the super-aligned carbon nanotube array consists of theplurality of carbon nanotubes, and the plurality of carbon nanotubes areparallel to each other and perpendicular to the substrate. A height ofthe super-aligned carbon nanotube array is about 200 μm to about 400 μm.

The super-aligned carbon nanotube array formed under the aboveconditions is essentially free of impurities such as carbonaceous orresidual catalyst particles. The plurality of carbon nanotubes can beobtained by removing the substrate.

In one embodiment, the diameter of each of the plurality of carbonnanotube is about 20 to 30 nanometers, and the length of each of theplurality of carbon nanotube is about 300 micrometers.

In the step 2, the plurality of carbon nanotubes are pre-oxidized.

After the plurality of carbon nanotubes are pre-oxidized, a plurality ofsites with negative charge, such as oxygen-containing functional groups,can be formed on surfaces of the plurality of carbon nanotubes. A mannerof the pre-oxidation of the plurality of carbon nanotubes is notlimited. In one embodiment, the plurality of carbon nanotubes can beheated for a period of time in an atmosphere of oxygen, carbon dioxideor air; the carbon nanotubes can be oxidized by oxygen, carbon dioxideor air. In another embodiment, the plurality of carbon nanotubes areimmersed into an oxidizing solution such as hydrogen peroxide or a acidsolution for a period of time; the carbon nanotubes can be oxidized bythe oxidizing solution. The acid solution can be a strong acid such as asulfuric acid (H₂SO₄), a hydrochloric acid (HCl) or a nitric acid(HNO₃). What should be noted is that, the carbon nanotubes can be etchedor corroded by a strong oxidizing agent such as oxygen or a strong acid,and a plurality of defect sites can be formed on surfaces of theplurality of carbon nanotubes. In one embodiment, the plurality ofdefect sites refer to a plurality of pores on surfaces of the pluralityof carbon nanotubes.

In one embodiment, the method of pre-oxidizing the plurality of carbonnanotubes comprises: placing the super-aligned carbon nanotube array ofthe step 1 in air; heating the super-aligned carbon nanotube array to550° C. for about 30 minutes. After the pre-oxidization process, theplurality of pores are formed on surfaces of the plurality of carbonnanotubes.

In the step 3, the plurality of pre-oxidized carbon nanotubes aredispersed in a solvent via a sonication-assisted method, and a firstsuspension is formed.

After the plurality of carbon nanotubes are pre-oxidized, a wettabilityand dispersity of the plurality of carbon nanotubes can be increased.Therefore, the pre-oxidized carbon nanotubes can be easily and uniformlydispersed in the solvent.

The solvent can be an organic solvent, water, or a mixed solvent oforganic solvent and water. In order to prevent the transition metaloxyacid radicals from reacting with other substances in the step S4, inone embodiment, the solvent is deionized water. In addition, when thesolvent is water, the transition metal oxyacid radicals can react withthe pre-oxidized carbon nanotubes in water. Therefore the batteryelectrode prepared in the step S5 can comprise the structural water. Thestructural water facilitates the fast charge transfer andelectrochemical reactivity.

Since the plurality of carbon nanotubes are easily agglomerated inwater, the plurality of carbon nanotubes can be dispersed uniformly in awater-miscible organic solvent first, and then the water-miscibleorganic solvent is gradually replaced with water. In this way, thesolvent of the first suspension can be water.

In one embodiment, after being pre-oxidized, the plurality of carbonnanotubes are dispersed uniformly in ethanol first, and then the ethanolis gradually replaced with the deionized water.

In the step S4, a material containing transition metal oxyacid radicalsis dispersed in the first suspension, and a second suspension is formed.

The material containing transition metal oxyacid radicals can be atransition metal oxyacid or a transition metal oxyacid salt. A valenceof a transition metal element in the material must be higher than thatof the transition metal element in the transition metal oxide to beprepared. Wherein, the transition metal oxyacid can be selected from agroup consisting of permanganic acid (HMnO₄), titanic acid (H₄TiO₄),chromic acid (H₂CrO₄), dichromic acid (H₂Cr₂O₇), ferric acid (H₂FeO₄),cobalt acid (H₃CoO₄), molybdenum acid (H₂MoO₄), vanadic acid (H₃VO₄),and combinations thereof. The transition metal oxyacid salt can beselected from a group consisting of permanganate, titanate, chromate,dichromate, ferrate, cobaltate, molybdate, vanadate, and combinationsthereof. The transition metal oxyacid salt can be potassium permanganate(KMnO₄), potassium titanate (K₄TiO₄), potassium chromate (K₂CrO₄),potassium dichromate (K₂Cr₂O₇), potassium ferrate (K₂FeO₄), potassiumcobaltate (K₃CoO₄), potassium molybdate (K₂MoO₄), potassium vanadate(K₃VO₄).

The material containing transition metal oxyacid radicals can be addedto the first suspension directly. Alternatively, a suspension of thematerial is formed first and then is mixed with the first suspension.The material containing transition metal oxyacid radicals can bedispersed in the first suspension uniformly by stirring. The transitionmetal oxyacid radicals become transition metal oxyanions in the firstsuspension. During the process of stirring, a redox reaction between thetransition metal oxyanions and the plurality of sites with negativecharge occurs to form a plurality of transition metal oxidenanoparticles. The plurality of transition metal oxide nanoparticles arechemically combined with the plurality of carbon nanotubes through C—O-Mbonds. The C—O-M bond is formed by an oxygen atom bonded between a metalatom of the transition metal oxide and a carbon atom of the carbonnanotube.

The redox reaction between the transition metal oxyanions and thepre-oxidized carbon nanotubes needs no heating and can be carried out atroom temperature. Moreover, during the redox reaction, the transitionmetal oxyanions can be uniformly contacted with the plurality ofpre-oxidized carbon nanotubes by continuously stirring. Thereby theplurality of transition metal oxide nanoparticles produced can beuniformly distributed in the battery electrode.

In one embodiment, the material containing transition metal oxyacidradical is KMnO₄. The KMnO₄ is added into the first suspension to form amixed solution. The mixed solution is stirred by a magnetic bar for 1day to 8 days at room temperature. During the process above, the KMnO₄reacts with the plurality of pre-oxidized carbon nanotubes to produce aplurality of MnO₂ nanoparticles on surfaces or in pores of the pluralityof carbon nanotubes.

In the step S5, the solvent is removed from the second suspension. Thesecond suspension is dried to form the battery electrode.

The solvent in the second suspension can be filtered out by vacuumfiltration. A remaining substance comprises the plurality of carbonnanotubes and the plurality of transition metal oxide nanoparticles. Theplurality of carbon nanotubes are entangled and closely combined witheach other to form a free-standing and flexible film structure. Theplurality of transition metal oxide nanoparticles are uniformlydistributed in the film structure.

In one embodiment, the deionized water in the second suspension isremoved by vacuum filtration, and a composite film structure comprisingthe plurality of carbon nanotubes and the plurality of MnO₂nanoparticles is obtained. The composite film structure is free-standingand highly conductive, therefore it can be used directly as a batteryelectrode, and no extra binders, conductive agents and currentcollectors are needed.

FIG. 2-FIG. 11 shows a series of characterization of the composite filmstructure prepared above. Wherein, the pristine CNT is the carbonnanotubes without any treatment, the air-CNT is the carbon nanotubesoxidized by the air, and the MnO₂/aCNT is a product of the redoxreaction between the air-CNT and the KMnO₂.

Referring to FIG. 2, the surface of the pristine CNT is smooth. Thesurface of the air-CNT is corroded and formed with defective sites.During the reaction of the KMnO₄ with the air-CNT, nanoparticles areformed on the surfaces of the carbon nanotubes.

The nanoparticles on the surfaces of the carbon nanotubes are furtheridentified as δ-MnO₂ referring to FIG. 3 and FIG. 4. As is shown in FIG.3, the strong peak at around 26° in the XRD pattern is attributed to the(002) crystal planes of the graphite lattice in the air-CNT. Other peaksmatch well with those of birnessite-type δ-MnO₂ phase. An interplanard₁₀₀-spacing of 0.25 nm observed in MnO₂ nanocrystals (As is shown inFIG. 3 inset) is also consistent with literature report for monoclinicbirnessite-type δ-MnO₂. FIG. 4 demonstrated the core level bindingenergy for Mn 2p peaks. The binding energy for Mn 2p_(3/2) and Mn2p_(1/2) are observed at 642.0 and 653.5 eV, which are close to Mn⁴⁺oxidation state.

As is shown in FIG. 5 and FIG. 5 inset, a plurality of pores are formedon the surfaces of the air-CNT, and sizes of the pores mainlyconcentrate at 3.7 nm and 62 nm. However, after the redox reactionbetween the air-CNT and the KMnO₄, a specific surface area of theair-CNT decreases and the pores of 3.7 nm almost disappear. Therefore,the MnO₂ nanoparticles are formed on surfaces of the air-CNT and in thepores of the air-CNT.

As is shown in FIG. 6, a size of the MnO₂ nanoparticles is about 10 nm.The ultrathin MnO₂ nanoparticles are either uniformly anchored onsurfaces of the air-CNT or encapsulated between adjacent nanotubes. Noextra MnO₂ nanoparticles agglomerated outside the CNT bundles.

As is shown in FIG. 7, the MnO₂/aCNT composite film structure isfree-standing and flexible, and can be used directly as a batteryelectrode.

As is shown in FIG. 8, two Raman peaks locate at 1348 cm⁻¹ (D peak) and1582 cm⁻¹ (G peak) in the Raman spectra of the air-CNT and the MnO₂/aCNTcomposite film structure. A relative value of an intensity of a D peakrepresents an amount of sp³ carbon atoms. That is, a six-membered ringof the multi-walled carbon nanotube is destroyed, and an destroyedlocation of the six-membered ring can be an oxidation site. The relativevalue of the intensity of a G peak represents an amount of sp² carbonatoms. That is, the six-membered ring of the multi-walled carbonnanotube is intact and not destroyed. It is shown that the process ofproducing MnO₂ nanoparticles basically retains the structure of theair-CNT. An increased intensity ratio of D peak to G peak suggests ahigher defect concentration of the air-CNT due to KMnO₄ oxidation. Inaddition, both the Raman peak at 634 cm⁻¹ of the MnO₂ powder and theRaman peak at 650 cm⁻¹ of the MnO₂/aCNT composite film structure arecorrespond to a stretching vibration of the Mn—O bond. A frequencydifference of MnO₂ (634 cm⁻¹) and the MnO₂/aCNT composites (650 cm⁻¹)can be correlated to shorter Mn—O chemical bonds due to the locallattice distortion by K⁺ incorporation into a interlayer region.

As is shown in FIG. 9, a presence of K⁺ in MnO₂/aCNT composite filmstructure is further indicated by two peaks located at 292.9 and 295.5eV related to K 2p_(3/2) and K 2p_(1/2) levels in XPS spectra. Excess K⁺in the interlayer region can stabilize a two-dimensional structure ofMnO₂ and improve a cycling stability of the MnO₂/aCNT electrode.

Referring to FIG. 10(a), for the MnO₂ powder, three peaks located at529.6 eV, 530.8 eV and 532.2 eV are assigned to oxide (Mn—O bonds), andhydroxide (Mn—O—H bonds) and structural water (H—O—H bonds),respectively. Referring to FIG. 10(b), for the air-CNT, two peakslocated at 530.8 eV and 532.2 eV are ascribed to C═O and C—O bonds,respectively. Referring to FIG. 3e , for the MnO₂/aCNT composite filmstructure, four peaks appearing at 529.6 eV, 530.8 eV, 531.5 eV and532.2 eV, correspond to oxide (Mn—O—Mn), hydroxide (Mn—O—H)/C═O bonds,C—O—Mn bonds and structural water (H—O—H)/C—O bonds, respectively.Wherein, compared with the MnO₂ powder and the air-CNT, an extra peak of531.5 eV of the MnO₂/aCNT composite film structure indicates a largefraction of C—O groups can come from the C—O—Mn bonds at an interfacebetween the air-CNT and the MnO₂. Oxygen-containing groups on carbontemplates can capture and react with various metal anions (such as VO₃⁻, MnO₄ ⁻, MoO₄ ⁻), and metal oxide can be produced. The MnO₂nanoparticles and the carbon nanotubes are chemically hybridized witheach other through the C—O—Mn bonds. The robust interaction can preventthe active materials detaching from the carbon nanotubes during cyclesand agglomerating on the surface of the carbon nanotubes.

Referring to FIG. 11, water contained in the MnO₂/aCNT composite filmstructure can be divided into physically adsorbed water and chemisorbedstructural water, wherein the former can be evaporated at 100° C. andthe latter can be removed at 100-300° C. The MnO₂/aCNT composite filmstructure suffers a serious mass loss at the temperature in a range fromabout 300° C. to about 500° C., as the air-CNT of the MnO₂/aCNTcomposite film structure become gas due oxidation reaction. A lastremaining material is the MnO₂ nanoparticles. In the MnO2/aCNT compositefilm structure, a mass fraction of the MnO₂ nanoparticles is estimatedto about 50 wt %, the mass fraction of the air-CNT is about to 42 wt %,and the mass fraction of the structural water is estimated to a moderateamount of 4-7 wt %. The structural water is supposed to facilitate thefast charge transfer and electrochemical reactivity.

In summary, the MnO₂/aCNT composite film structure prepared in theembodiment is a flexible and self-supporting structure. Wherein, MnO₂ isa 6 phase. The size of the MnO₂ nanoparticles is less than 10nanometers. The MnO₂ nanoparticles are uniformly distributed on thesurface of the carbon nanotubes or in the pores of the carbon nanotubes,and the MnO₂ nanoparticles are tightly bound to the carbon nanotubes byC—O—Mn chemical bonds. In addition, the MnO₂/aCNT composite filmstructure also comprises K⁺ and structural water. The K⁺ and structuralwater can be beneficial to stabilize the MnO₂/aCNT composite filmstructure and prevent the MnO₂ nanoparticles from detaching from thesurfaces of the carbon nanotubes under high current charge anddischarge.

A hybrid energy storage device is further provided in one embodiment.Wherein, the hybrid energy storage device comprises two electrodes, andone of the two electrodes of the hybrid energy storage device can be thebattery electrode as mentioned above.

In order to further illustrate a performance of the battery electrodeand the hybrid energy storage device, two comparative embodiment and oneblank embodiment are utilized in the present disclosure.

Embodiment 1

Preparing the MnO₂/aCNT electrode: a super-aligned carbon nanotube arrayis placed in air and heated to 550° C. at a rate of 15° C. per minute.Then, the super-aligned carbon nanotube array is heated at 550° C. for30 minutes to prepare the air-CNT. 100 mg air-CNT are dispersed into thedeionized water via ultra-sonication to prepare a carbon nanotubesuspension. Then 1.0 g KMnO₄ are added into the carbon nanotubesuspension to obtain a mixed solution. The mixed solution is stirred bymagnetic bar for 6 days at room temperature. The deionized water in thesecond suspension is removed via vacuum filtration to obtain theMnO₂/aCNT electrode. The electrode is free-standing and no extra currentcollectors or binders are needed.

Preparing a battery: a coin-type (CR 2016) half-cell is assembled in anAr-filled glove box. The MnO₂/aCNT electrode is treated as a workingelectrode and a lithium foil is worked as a reference electrode. Apolypropylene film (Celgard 2400) is employed as a separator. Anelectrolyte is 1M LiPF₆ in a 1:1 weight ratio of EC:DEC.

Comparative Embodiment 1

Preparing the MnO₂ electrode: a conventional MnO₂ electrode slurry isprepared by mixing MnO₂ powder, carbon black, and PVDF inN-methylpyrrolidone (NMP) solvent at a weight ratio of 5:4:1. Then, theslurry is coated on a copper foil surface and dried in vacuum.

Preparing the battery: a coin-type (CR 2016) half-cell is assembled inan Ar-filled glove box. The MnO₂ electrode is treated as a workingelectrode and a lithium foil is worked as a reference electrode. Apolypropylene film (Celgard 2400) is employed as a separator. Anelectrolyte is 1M LiPF₆ in a 1:1 weight ratio of EC:DEC.

Comparative Embodiment 2

Preparing the MnO₂/rCNT electrode: a disordered carbon nanotube array ispurchased. Carbon nanotubes of the disordered carbon nanotube array arerandomly grown and oriented. The commercial randomly oriented carbonnanotubes are denoted as rCNT. The rCNT are treated following the sameprocedure as the embodiment 1 to prepare MnO₂/rCNT composite materials.The MnO₂/rCNT composite materials are not free-standing structure andcan not serve as a conductive scaffold, so the current collector,conductive agent and binder are needed. The MnO₂/rCNT compositematerials, carbon black, and PVDF are mixed in N-methylpyrrolidone (NMP)solvent to obtain a slurry. Then, the slurry is coated on a copper foilsurface and dried in vacuum.

Preparing the battery: a coin-type (CR 2016) half-cell is assembled inan Ar-filled glove box. The MnO₂/rCNT electrode is treated as a workingelectrode and a lithium foil is worked as a reference electrode. Apolypropylene film (Celgard 2400) is employed as a separator. Anelectrolyte is 1M LiPF₆ in a 1:1 weight ratio of EC:DEC.

Blank Embodiment

Preparing the air-CNT electrode: the air-CNT are dispersed into thedeionized water to obtain a carbon nanotube suspension. Then, thedeionized water of the suspension is removed via vacuum filtration toprepare the air-CNT electrode. The air-CNT electrode without MnO₂coating is used as a blank sample to identify a contribution of thecarbon nanotubes in the MnO₂/aCNT electrode.

Preparing the battery: a coin-type (CR 2016) half-cell is assembled inan Ar-filled glove box. The air-CNT electrode is treated as a workingelectrode and a lithium foil is worked as a reference electrode. Apolypropylene film (Celgard 2400) is employed as a separator. Anelectrolyte is 1 M LiPF₆ in a 1:1 weight ratio of EC:DEC.

A series of tests are performed on the four electrodes, as is shown inFIG. 12-FIG. 17.

Referring to FIG. 12, the rCNT are highly agglomerated together, so therCNT can not work as a conductive network (FIG. 12a ). Although thecontinuous spinning-like air-CNT can no longer maintain a super-alignedstructure after ultrasonication, but the air-CNT can still be used as athree-dimensional conductive scaffold (FIG. 12b ). An aggregation ofconductive agents and MnO₂ particles can be seen in the MnO₂ electrodebecause the MnO₂ particles are prone to aggregate into a spherical shape(FIG. 12c ). Since the rCNT can not serve as a conductive scaffold, theMnO₂/rCNT electrode displays a similar morphology to the conventionalMnO₂ electrode. The morphology of the air-CNT is similar to that of theMnO₂/aCNT electrode. Thus, the MnO₂ nanoparticles is uniformly coatedonto carbon templates without aggregation, which is in accordance withTEM images discussed above.

Referring to FIG. 13, compared with the MnO₂ electrode and the MnO₂/rCNTelectrode, the MnO₂/aCNT electrode exhibits three unique features: (1) avoltage drop (AU, when charging is switched to discharging) is small(ΔU_(1st)=0 V, ΔU_(2nd)=0.22 V, ΔU_(50th)=0.08 V), reflecting a lowcontact resistance between the active materials and the carbonnanotubes. The low contact resistance can be ascribed to an intimatechemical contact by C—O—Mn bonds. (2) For the MnO₂/aCNT electrode, aretention rate of an initial capacity after the first cycle is as highas 70%. While for the other two electrodes, the retention rate of aninitial capacity after the first cycle is less than 50%. Compared withthe other two electrodes, the advantage of the MnO₂/aCNT electrode maystem from that a solid electrolyte interface (SEI) film can be easier tobe formed. In the MnO₂/aCNT electrode, oxygen-containing functionalgroups and smooth surfaces of carbon nanotubes can facilitate aformation of the SEI film. (3) During the 50th cycle, the capacity ofthe MnO₂/aCNT electrode is increased compared to the second cycle. Twophenomena exist in the three electrodes: on the one hand, it usuallytakes 0.5 to 2 weeks for the electrolyte to infiltrate the electrodecompletely. At this time, all MnO₂ nanoparticles are activated to becomeactive materials in reaction. Therefore, all MnO₂ electrodes wouldexperience an initial capacity rise. On the other hand, due to apolarization of the lithium-ion batteries, irreversible changes such asa loss of active materials can result in a decrease of the dischargecapacity with an increase of the cycle number. The polarization of theMnO₂/aCNT electrode is small and the former is dominant, so the capacityof the first 50th cycle can rise; while the other two electrodes areseverely polarized and the latter is dominant, so the capacity continuesto decrease.

Referring to FIG. 14, the MnO₂/aCNT electrode delivers dischargecapacities of 1043.5 mAhg⁻¹ and 718.3 mAhg⁻¹ for the initial and secondcycle, respectively. The capacity keeps increasing after the 3^(rd)cycle, and a maximum capacity of 843.8 mAhg⁻¹ is achieved at the 70^(th)cycle. The capacity of the MnO₂/aCNT electrode even exceeds itstheoretical specific capacity (taking the 70^(th) cycle into anexample): the MnO₂/aCNT electrode contains three main components ofair-CNT, MnO₂ and structural water at a weight ratio of 42:50:8. Theair-CNT exhibits 175.3 mAhg⁻¹ in the 70^(th) cycle and the theoreticalcapacity of MnO₂ is 1233 mAhg⁻¹, so the upper limit of the dischargecapacity of the MnO₂/aCNT electrode is supposed to be 774 mAhg⁻¹ (175.3mAhg⁻¹×45%+1233 mAhg⁻¹×50%+0×5%=774 mAhg⁻¹). However, a dischargecapacity of 843.8 mAhg⁻¹ can be delivered in actual. The excess capacitymay stem from an interfacial capacitive-like lithium storage behaviorbetween the MnO₂ and the carbon nanotubes. The lithium-ions can becompensated by electrons on surfaces of the carbon nanotubes. Incomparison, the performances of the other three electrodes are poor. Forthe MnO₂/rCNT, the air-CNT and the MnO₂ electrodes, initial capacitiesof 1203.8, 835.4 and 616.5 mAhg⁻¹ degrade to 380.2, 182.7 and 145.4mAhg⁻¹ after 20 cycles, respectively. The MnO₂/aCNT electrode exhibitssuperior cycle stability compared to the other three electrodes.

Referring to FIG. 15, the MnO₂/aCNT electrode delivers a high reversibledischarge capacity of 671.9, 649.1, 619.2, 539.9 and 395.8 mAhg⁻¹ atstepwise charge rates of 0.5, 1, 2, 5 and 10 Ag⁻¹, respectively. Whenthe current rate is returned at 0.5 Ag⁻¹, the discharge capacity of theelectrode is approximately 648.1 mAhg⁻¹. However, the other twoelectrodes exhibit poor rate performance: the capacity of the other twoelectrodes is much lower than that of the MnO₂/aCNT electrode at 0.5Ag⁻¹; when the charge rate exceeds 2 Ag⁻¹, the capacity is rapidlydecayed. The active materials of the MnO₂ electrode and the MnO₂/rCNTelectrode are easy to aggregate and detach from the current collector athigh charge rates. Therefore, the rate performance of the other twoelectrodes is poor. Compared to the other two electrodes, the MnO₂/aCNTelectrode also exhibits superior rate performance.

Referring to FIG. 16, at 2 Ag⁻¹, the capacity of the MnO₂/aCNT electrodekeeps increasing after the 3^(rd) cycle and reaches a maximum at150^(th) cycle. In addition, the MnO₂/aCNT electrode exhibits lowcapacity fading rates from 800^(th) to 1000^(th) cycle. The MnO₂/aCNTelectrode shows excellent long cycle performance at the highcharge/discharge current.

The outstanding rate performance and excellent long cycle stability ofthe MnO₂/aCNT electrode originate from the following four factors: (1) Athickness of the MnO₂/aCNT electrode is ultrathin. Therefore, a largefraction of surface atoms can be allowed to be contact with theelectrolyte, and a diffusion path of lithium ions can be shortened and adiffusion time of lithium-ions can be reduced. (2) The robust contactbetween the active materials and the carbon nanotubes can prevent theactive materials from detaching from the electrode during the fastcharge/discharge process. Furthermore, the low contact resistance due tothe strong adhesion can prevent the polarization during the conversionreaction. (3) The free-standing CNT scaffold serves as 3D continuouselectron pathways. These 3D continuous electron pathways can efficientlytransfer electron and alleviate polarization. (4) The K⁺ and structuralwater can stabilize the electrode structure during cycles.

The outstanding rate performance of the MnO₂/aCNT electrode is alsorelated to its capacitive-like storage behavior. Referring to FIG. 16inset, in 2^(nd) cycle, several clear discharge/charge plateaus can beseen. The 2nd cycle is corresponding to a battery-type diffusionprocess. In 500^(th) cycle, only one obvious discharge plateaus can beidentified and others become obscure. In 1000^(th) cycle, even the lastdischarge plateaus nearly disappears, and the charge/discharge curvebecame linear, indicating that the capacitive-like storage behaviordominates in this cycle. The capacitive-like storage behavior can bestrengthened as the number of cycles increases.

As the capacitive-like storage behavior has already revealed by aboveexperiments and results. A ratio of a contribution of thecapacitive-like storage behavior is further quantified by FIG. 17.

Referring to FIG. 17a , a power law relationship between a currentresponse (i) and a scan rate (v) can be used to reveal the storagemechanism: i=av^(b), wherein a and b are adjustable parameters. When bequals 1, the response current is proportional to the scan rate,indicating a capacitive process. When b equals to 0.5, the responsecurrent satisfies Cottrell's equation, reflecting a diffusion-controlledprocess. Referring to FIG. 17b , the b value is between 0.5 and 1.0,implying that a process of lithium-ions storage is a hybrid ofbattery-type diffusion and capacitive-like storage.

As is shown in the FIG. 17c , a shaded area presents the contribution ofcapacitive charge storage at the scan rate of 20 mV s⁻¹. Thecontribution of capacitive charge storage (460 C g⁻¹) accounts for 65.6%of the total capacity (700 C g⁻¹). The contribution of battery diffusioncontrol storage is only 240 C·g⁻¹. The MnO₂/aCNT electrode is mainlycontrolled by the capacitive-like behavior at the scan rate of 20 mVs⁻¹.

A valence state of manganese can be characterized by an interval betweentwo main peaks of Mn 3s XPS spectrum. Referring to FIG. 17d , intervalsat 3.0 and 0.01V are 5.83 and 6.25 eV, corresponding to Mn valencies of2.4 and 2.0, respectively. Since the capacitive charge storage dominatesat the scan rate of 20 mV·s⁻¹, a change in the valence state ofmanganese before and after discharge is small. The redox reactionbetween manganese and lithium contributes only 222 C·g⁻¹. This result isin agreement with the above kinetic analysis.

The MnO₂/aCNT electrode exhibits a hybrid characteristic of capacitorand battery. The capacitive characteristic dominates. So a large amountof energy can be rapidly received or delivered in a short time. Thehybrid energy storage device using the MnO₂/aCNT electrode can achieveadvantages of high energy density and high power density simultaneously.Wherein, a lithium-ion storage mechanism can be explained as thefollowings: on one hand, a part of lithium-ions are diffused into theactive materials, and this process is accompanied by the redox reactionof lithium-ions and active materials; on the other hand, the other partof lithium-ions are compensated by extra electrons on surfaces of thecarbon nanotubes and stored in the interface between the activematerials and the carbon nanotubes.

The battery electrode provided in this disclosure comprises thefollowing advantages.

Firstly, the transition metal oxide nanoparticles in the batteryelectrode are uniformly attached to the carbon nanotubes through C—O-Mchemical bonds. Therefore, the transition metal oxide nanoparticles arenot easy to agglomerate and detach from surfaces of the carbonnanotubes.

Secondly, the carbon nanotubes character in high elasticity andconductivity, so they can serve as conductive frameworks for thetransition metal oxide nanoparticles and buffer a volume change of theactive materials during the fast charge/discharge process.

Thirdly, the battery electrode further comprises structural water andalkali metal ions. Structural water and alkali metal ions can help thestability of the electrode during cycles.

Fourthly, the battery electrode need no current collector, conductiveagent, or binder. The weight of the battery electrode can be reduced.

Fifth, the diameter of the transition metal oxide particles is 10nanometers on average and the battery electrode is ultrathin. Thediffusion path of lithium-ions can be shortened and the diffusion timeof lithium-ions can be reduced.

The method for preparing the battery electrode provided in thedisclosure comprises the following advantages.

Firstly, the transition metal oxyacid radicals react with theair-oxidized carbon nanotubes to produce the transition metal oxidenanoparticles. The transition metal oxide nanoparticles are tightlyattached to the carbon nanotubes through chemical bonds. In addition,due to continuous stirring and mixing during the reaction, thetransition metal oxide can be uniformly distributed in the batteryelectrode.

Secondly, the method can be carried out at room temperature. No reagentsother than water or ethanol are needed. No toxic substances are producedduring the preparation. Therefore, the method is simple andenvironmentally friendly.

The hybrid energy storage device provided in this disclosure can achieveadvantages of battery and supercapacitor simultaneously. The hybridenergy storage device characters in low capacity fading rate,outstanding cycle stability and excellent rate performance.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. A battery electrode, comprising: a plurality ofcarbon nanotubes, a plurality of transition metal oxide nanoparticles,structural water, and potassium ions; wherein the plurality oftransition metal oxide nanoparticles are chemically bonded to theplurality of carbon nanotubes through carbon-oxygen-metal (C—O-M)linkages, and the metal is a transition metal element, wherein surfacesof the plurality of carbon nanotubes define a plurality of pores, andthe potassium ions are incorporated into an interlayer region of theplurality of transition metal oxide nanoparticles.
 2. The batteryelectrode of claim 1, wherein the plurality of carbon nanotubes are aplurality of multi-walled carbon nanotubes.
 3. The battery electrode ofclaim 1, wherein the plurality of carbon nanotubes are entangled andcombined with each other to form a free-standing film structure.
 4. Thebattery electrode of claim 1, wherein the transition metal oxide isselected from a group consisting of manganese dioxide, titanium dioxide,ferroferric oxide, chromic oxide, cobaltosic oxide, molybdenum dioxide,vanadium dioxide, and combinations thereof.
 5. The battery electrode ofclaim 1, wherein the plurality of transition metal oxide nanoparticlesare deposited on surfaces of the plurality of carbon nanotubes or in theplurality of pores.
 6. The battery electrode of claim 1, wherein theplurality of pores having a size of about 3.7 nm or about 62 nm.
 7. Ahybrid energy storage device, comprising: a battery electrode comprisinga plurality of carbon nanotubes, a plurality of transition metal oxidenanoparticles, structural water, and potassium ions; wherein theplurality of transition metal oxide nanoparticles are chemically bondedto the plurality of carbon nanotubes through carbon-oxygen-metal (C—O-M)linkages, the potassium ions is incorporated into an interlayer regionof the plurality of transition metal oxide nanoparticles, and the metalbeing a transition metal element.
 8. The hybrid energy storage device ofclaim 7, wherein the plurality of carbon nanotubes are a plurality ofmulti-walled carbon nanotubes.
 9. The hybrid energy storage device ofclaim 7, wherein the plurality of carbon nanotubes are entangled andcombined with each other to form a free-standing film structure.
 10. Thehybrid energy storage device of claim 7, wherein the transition metaloxide is selected from a group consisting of manganese dioxide, titaniumdioxide, ferroferric oxide, chromic oxide, cobaltosic oxide, molybdenumdioxide, vanadium dioxide, and combinations thereof.
 11. The hybridenergy storage device of claim 7, wherein surfaces of the plurality ofcarbon nanotubes define a plurality of pores.
 12. The hybrid energystorage device of claim 11, wherein the plurality of transition metaloxide nanoparticles are deposited on surfaces of the plurality of carbonnanotubes or in the plurality of pores.
 13. The hybrid energy storagedevice of claim 7, wherein the plurality of pores having a size of about3.7 nm or about 62 nm.
 14. The hybrid energy storage device of claim 7,wherein the potassium ions are incorporated into the interlayer regionof the plurality of transition metal oxide nanoparticles, thereby locallattices formed by the carbon-oxygen-metal (C—O-M) linkages aredistorted.